Refactored plugin-loader with improved dependency resolution

With the new implementation the plugins don't have to be listed in any
special order, dependencies are properly resolved.  The order only
matters if two plugins provide the same feature.

src/libcharon/plugins/stroke/stroke_list.c

@@ -1364,6 +1364,7 @@ static void list_plugins(FILE *out)
 				free(str);
 			}
 		}
+		list->destroy(list);
 	}
 	enumerator->destroy(enumerator);
 }